1. Field of the Invention
The present invention relates to a document transport device for transporting sheets such as documents and the like used in a copy machine, a scanner, a facsimile device, and the like, and more particularly, to a double-sided document transport device for reading the image information recorded on both the sides of a sheet-like document from each side thereof.
2. Description of the Related Art
Heretofore, many of sheet-like documents such as a printed matter and the like often have image information printed or recorded on both the sides thereof. There have been proposed various types of sheet transport devices which can read the image information recorded on both the sides of a document. Many of the sheet transport devices used to read a document read image information by a raster scan system using a one-dimensional image sensor by moving a reading side of the document on a platen glass. To read the image information recorded on both the sides of a document by this type of the sheet transport device, after one reading side of the document is read, the document must be moved with the other reading side thereof in confrontation with the platen glass. For this purpose, a device for automatically reading both the sides of a document must be provided with a reverse mechanism for reversing a reading side after one side of the document is read.
Incidentally, an ordinary sheet transport device used to read a document transports various types of documents as subjects for transportation. Accordingly, the ordinary sheet transport device cannot entirely eliminate the danger that a document is jammed in a transport mechanism. In a sheet transport device having a reverse mechanism for reversing a document, a transport path is more complex than that of a sheet transport device for reading only one side of a document, and thus danger of the occurrence of jam is increased. To cope with this problem, there have been proposed sheet transport devices provided with various countermeasures against jam.
For example, there is proposed a double-sided document transport device which transports documents in such a manner that they are separated from a bundle of a plurality of documents one by one and stop at a reading position (for example, refer to Japanese Patent Publication No. 09-86807A).
There is a description of separation of a discharge roller in the conventional double-sided document transport device. However, since the discharge roller is disposed to a lower side and remains exposed from a document guide, the document transport device has a drawback in that when it is intended to guide a document to a reverse path after the image information recorded on the front side thereof is read, a document having been discharged may become entangled.
FIGS. 1A and 1B show the drawback. In the figures, reference numeral 400 denotes a bundle of documents having been read and discharged, and reference numeral 401 denotes a document discharged finally among them. As shown in FIG. 1A, the rear end of a document having been discharged is often in contact with a lower roller 501 for a while. Here, attention must be paid to that the lower roller 501 is a discharge roller and is composed of a material such rubber and the like.
When the processing of the front side of a document 601, which is being read at present, is finished and the discharge roller 501 begins to rotate reversely to read the rear side of the document, the document 401 discharged finally becomes entangled in the device together with the document 601 as shown in FIG. 1B. As a result, there occur drawbacks such as jam, irregularly stacked documents, and the like.
Further, the above conventional example describes nothing as to methods of separating the discharge roller and reversing the rotating direction thereof.
To overcome the above drawback, for example, a discharge roller is disposed on an upper side, and a pinch roller is disposed under the discharge roller to press a document against the discharge roller. This type of the pinch roller does not require feed force and can be molded of polyacetal resin and the like. Even if the pinch roller is rotate reversely, there is not a possibility that a discharged document becomes tangled therewith, thereby the above drawback can be overcome.
However, a new problem is caused by the discharge roller because it is disposed on the upper side. It is most effective to permit the vicinity of a discharge section to be opened and closed to take out a jammed document, and further it is preferable to separate the discharge roller from the pinch roller when the vicinity of the discharge section is opened.
In contrast, it is effective to use the shaft of the discharge roller also as a shaft for opening and closing the discharge section. In this case, however, since the position of the discharge roller cannot be changed to execute an open/close operation, it is difficult to separate the discharge roller from the pinch roller.
That is, there arises a new problem as to how drive force is transmitted to the discharge roller from the outside when the discharge section is closed while automatically separating the discharge roller from the pinch roller when the discharge section opened.